Move FrameData::clear into BitmapImage.cpp.

It is declared in BitmapImage.h, so it doesn't belong in ImageSkia.cpp.

BUG=241546

Review URL: https://chromiumcodereview.appspot.com/21020007

git-svn-id: svn://svn.chromium.org/blink/trunk@155109 bbb929c8-8fbe-4397-9dbb-9b2b20218538